Methods and apparatus for controlling operation of photovoltaic power plants

ABSTRACT

A photovoltaic power plant includes a photovoltaic inverter that converts direct current generated by solar cells to alternating current. The output of the photovoltaic inverter is provided to a point of interconnection to a power grid. A meter at the point of interconnection may be read to detect the output of the photovoltaic inverter at the power grid. The photovoltaic power plant includes a plant controller with a state machine. The plant controller is configured to adjust setpoints of the photovoltaic inverter to control the output of the photovoltaic power plant. The plant controller is also configured to soft start and soft stop automatic voltage regulation (AVR) of the photovoltaic power plant to prevent perturbing the AVR.

TECHNICAL FIELD

The present invention relates to photovoltaic power plants.

BACKGROUND

Photovoltaic power plants employ photovoltaic systems to generateelectricity from solar radiation. A photovoltaic system may comprisearrays of solar panels, with each solar panel comprising interconnectedsolar cells. A solar cell includes P-type and N-type diffusion regions.Solar radiation impinging on the solar cell creates electrons and holesthat migrate to the diffusion regions, thereby creating voltagedifferentials between the diffusion regions. In a backside contact solarcell, both the diffusion regions and the metal contact fingers coupledto them are on the backside of the solar cell. The contact fingers allowan external electrical circuit to be coupled to and be powered by thesolar cell.

A photovoltaic inverter converts direct current generated by the solarcells to alternating current suitable for coupling to a power grid at apoint of interconnection (POI). The output of the photovoltaic powerplant at the POI, such as reactive power, real power, and power factor,are controlled to be within a range of specified values to meetrequirements. Embodiments of the present invention pertain to methodsand apparatus for controlling the operation of the photovoltaic powerplant to control the photovoltaic power plant output at the POI or otherdelivery node.

BRIEF SUMMARY

In one embodiment, a method of controlling operation of a photovoltaicpower plant comprises generating direct current using a plurality ofsolar cells. The direct current generated by the solar cells isconverted to alternating current using a photovoltaic inverter. Theoutput of the photovoltaic inverter is coupled to a power grid. Thephotovoltaic power plant is detected to generate a photovoltaic powerplant output that exceeds a maximum allowable output limit of thephotovoltaic power plant. In response to detecting that the photovoltaicpower plant generates a photovoltaic power plant output that exceeds themaximum allowable output limit of the photovoltaic power plant, asetpoint variable limit of the photovoltaic inverter is lowered from afirst inverter setpoint limit to a second inverter setpoint limit.

In another embodiment, a method of controlling operation of aphotovoltaic power plant comprises detecting that a real power output ofthe photovoltaic power plant is below a minimum real power generationlevel. In response to detecting that the real power output of thephotovoltaic power plant is below the minimum real power generationlevel, a reactive power setpoint of a photovoltaic inverter of thephotovoltaic power plant is ramped down to unity power factor.

In another embodiment, a photovoltaic power plant comprises a pluralityof solar cells, a photovoltaic inverter configured to convert directcurrent generated by the plurality of solar cells to alternatingcurrent, a meter configured to detect an output of the photovoltaicinverter at a point of interconnection to a power grid, and a plantcontroller comprising a state machine. The plant controller isconfigured to read the meter to detect the output of the photovoltaicpower plant at the point of interconnection in a first state ofoperation and to dynamically adjust a setpoint variable limit of thephotovoltaic inverter from a first inverter setpoint limit to a secondinverter setpoint limit when the output of the photovoltaic power plantexceeds a maximum allowable output.

In another embodiment, a photovoltaic power plant comprises a pluralityof solar cells, a photovoltaic inverter configured to convert directcurrent generated by the plurality of solar cells to alternatingcurrent, a meter configured to detect an output of the photovoltaicinverter at a point of interconnection to a power grid, and a plantcontroller. The plant controller is configured to read the meter todetect a real power output of the photovoltaic power plant at the pointof interconnection, to compare the real power output of the photovoltaicpower plant to a minimum real power generation level, and to ramp down areactive power setpoint of the photovoltaic inverter to unity powerfactor in response to detecting the real power output of thephotovoltaic power plant to be below the minimum real power generationlevel.

These and other features of the present invention will be readilyapparent to persons of ordinary skill in the art upon reading theentirety of this disclosure, which includes the accompanying drawingsand claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the subject matter may be derived byreferring to the detailed description and claims when considered inconjunction with the following figures, wherein like reference numbersrefer to similar elements throughout the figures.

FIG. 1 schematically shows components of a photovoltaic power plant inaccordance with an embodiment of the present invention.

FIG. 2 schematically shows additional components of the photovoltaicpower plant in accordance with an embodiment of the present invention.

FIG. 3 schematically shows further details of the photovoltaic powerplant in accordance with an embodiment of the present invention.

FIG. 4 shows a state diagram of a state machine for automatic voltageregulation of a photovoltaic power plant in accordance with anembodiment of the present invention.

FIGS. 5-7 show a flow diagram of a method of controlling operation of aphotovoltaic power plant in accordance with an embodiment of the presentinvention.

FIG. 8 shows a flow diagram of a method of controlling operation of aphotovoltaic power plant in accordance with an embodiment of the presentinvention.

FIG. 9 shows a flow diagram of a method of controlling operation of aphotovoltaic power plant in accordance with an embodiment of the presentinvention.

FIG. 10 shows a flow diagram of a method of controlling operation of aphotovoltaic power plant in accordance with an embodiment of the presentinvention.

FIG. 11 shows a flow diagram of a method of controlling operation of aphotovoltaic power plant in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION

In the present disclosure, numerous specific details are provided, suchas examples of apparatus, components, and methods, to provide a thoroughunderstanding of embodiments of the invention. Persons of ordinary skillin the art will recognize, however, that the invention can be practicedwithout one or more of the specific details. In other instances,well-known details are not shown or described to avoid obscuring aspectsof the invention.

FIG. 1 schematically shows components of a photovoltaic power plant 200in accordance with an embodiment of the present invention. Thecomponents of the photovoltaic power plant 200 shown in the example ofFIG. 1 include a plurality of combiner boxes 112, a plurality of solarpanels 114, and a photovoltaic inverter 110. A photovoltaic power plantmay include a plurality of photovoltaic inverters but only one is shownin FIG. 1 for clarity of illustration. A solar panel 114 compriseselectrically connected solar cells mounted on the same frame. In oneembodiment, each solar panel 114 comprises a plurality ofserially-connected backside contact solar cells 115. Front contact solarcells may also be employed. Only some of the backside contact solarcells 115 have been labeled in FIG. 1 for clarity of illustration.

A photovoltaic string comprises a plurality of serially-connected solarpanels 114 as in FIG. 1. A group of solar panels 114 is electricallyconnected to a combiner box 112, where the solar panels 114 areconnected in series. The combiner boxes 112 are electrically connectedsuch that that all solar panels 114 in the photovoltaic string areserially-connected. The output of the photovoltaic string iselectrically connected to the inverter 110, which converts directcurrent (DC) generated by the solar cells 115 to alternating current(AC) suitable for delivery to a utility power grid, for example.

FIG. 2 schematically shows additional components of the photovoltaicpower plant 200 in accordance with an embodiment of the presentinvention. FIG. 2 shows the inverters 110 described in connection withFIG. 1. The solar panels 114 are not shown in FIG. 2 for clarity ofillustration. In the example of FIG. 2, the components of thephotovoltaic power plant 200 are located in a photovoltaic substation210 and inverter pads 220.

An inverter pad 220 is a general area where inverters are located. Theinverter pads 220 are typically located away from the substation 210,far from the point of interconnection (POI) 204 with the power grid.Communication modules 201 allow for data communications between theinverters 110 and components located in the substation 210. An inverterpad 220 may also include additional components that are not specificallyshown in FIG. 2, such as analog to digital converters, digital to analogconverters, and other components for supporting the operation of theinverters 110.

In one embodiment, the operation of the photovoltaic power plant 200 iscontrolled using Supervisory Control and Data Acquisition (SCADA), withthe plant controller 202 serving as the central control computer. In oneembodiment, the inverters 110, the plant controller 202, and asubstation computer 203 communicate in accordance with the Modbus TCP/IPcommunications protocol. In that embodiment, the communication modules201 comprise Ethernet switches that provide data communication linksbetween components of the photovoltaic power plant 200. Monitoring andcontrol may also be performed by analog signaling, such as by providingindividual wiring for signals.

In the example of FIG. 2, a step-up transformer in an inverter pad 220steps-up the AC voltage output of an inverter 110 to a higher voltagefor distribution to the substation 210. Also in the example of FIG. 2, agenerator step-up (GSU) transformer in the substation 210 furthersteps-up the AC voltage received from the inverter pads 220 before beingcoupled to the POI 204 for distribution to the power grid (not shown). Asubstation computer 203 allows for control and monitoring of thesubstation 210. The substation computer 203 may be configured to controlprotective circuits and read the voltage at the POI 204 by way of ameter 205. The meter 205 may comprise a conventional electrical meter orother sensing element, e.g., an RMS transmitter.

A plant controller 202 in the substation 210 may comprise aspecial-purpose or general-purpose computer configured to facilitatecontrol of the voltage, power factor, real power, or reactive power at(or near) the POI 204. In the example of FIG. 2, the plant controller202 includes a state machine 400 for dynamically limiting the reactivepower, power factor of the photovoltaic power plant 200, and/or inverterterminal voltages, and for providing a soft start and soft stop ofautomatic voltage regulation.

In one embodiment, the plant controller 202 is configured to read thePOI meter 205 to detect an output (e.g., reactive power, power factor)of the photovoltaic power plant 200 at the POI 204 in a first state ofoperation and to dynamically adjust a setpoint variable limit of thephotovoltaic inverters 110 (e.g., inverter reactive power or powerfactor setpoint) from a first inverter setpoint limit to a secondinverter setpoint limit when the output of the photovoltaic power plant200 exceeds a maximum allowable output (e.g., maximum reactive power orpower factor).

In one embodiment, the plant controller 202 is configured to read thePOI meter 205 to detect a real power output of the photovoltaic powerplant 200 at the POI 204, to compare the real power output of thephotovoltaic power plant 204 to a minimum real power generation level,and to ramp down a reactive power setpoint of the photovoltaic inverters110 to unity power factor (i.e., 1.0 PF or zero reactive power) inresponse to detecting that the real power output of the photovoltaicpower plant 200 is below the minimum real power generation level. Theplant controller 202 may be further configured to ramp up the reactivepower setpoint of the photovoltaic inverters 110 to increase thereactive power output of the photovoltaic power plant 200 in response todetecting that the real power output of the photovoltaic power plant 200is above an enabling real power generation level that is higher than theminimum real power generation level.

The voltage at a point of interconnection may be controlled by automaticvoltage regulation (AVR). Generally speaking, AVR may involvemanipulation of reactive power export/import from the photovoltaic powerplant by manipulating reactive and/or power factor setpoints of theinverter to control the voltage at the point of interconnection with thedistribution, transmission, or other electrical connection to the grid.AVR may also be employed to coordinate operation of a photovoltaic powerplant with reactive power sources, such as static VAR compensators andcapacitive banks.

FIG. 3 schematically shows further details of the photovoltaic powerplant 200 in accordance with an embodiment of the present invention.

In the example of FIG. 3, the function blocks 301-313 are performed bythe plant controller 202. As can be appreciated, these functions may beimplemented in software, hardware, or combination of hardware/software.For example, the plant controller 202 may comprise a computer withhardware interfaces for analog input direct measurement and control,data communications (e.g., Ethernet network adapter), data acquisition(e.g., to receive signals) and control (e.g., to send control signals),and associated driver software. The plant controller 202 may utilizededicated processors or co-processors in combination with applicationsoftware to perform its functions. The plant controller 202 may also beemployed in a master-slave configuration with other controllers tocoordinate operation of the photovoltaic power plant 200 with reactivepower sources, such as static VAR compensators and capacitive banks.

In the example of FIG. 3, a global inverter reactive power setpointsignal is processed from function blocks 302-306. From the functionblock 306, an individual inverter reactive power setpoint signal is sentto each inverter 110. An inverter 110 exports/absorbs reactive powerbased on received inverter reactive power setpoint.

Referring to FIG. 3, the plant controller 202 receives a referencevoltage V_(REF) that is employed as a setpoint voltage for setting thevoltage at the POI 204. A summer 301 generates an error signal V_(ERR)based on the difference between the desired voltage at the POI 204 asindicated by the reference voltage V_(REF) and the voltage at the POI204 as measured by the meter 205 (V_(METER)).

In one embodiment, the reference voltage V_(REF) and the meter voltagereading V_(METER) are processed in the plant controller 202 as digitalsignals. These voltages may be converted to digital using an analog todigital converter (ADC), and then provided to the plant controller 202over a data communications network. As a particular example, thereference voltage V_(REF) and the meter voltage reading V_(METER) may beprovided to the plant controller 202 by way of ModbusTCP registers.Bounds checking may be performed on commands and inputs (including thereference voltage V_(REF) and the meter voltage reading V_(METER))received by the plant controller 202.

The reference voltage V_(REF), the meter voltage reading V_(METER), andother voltages/currents in the photovoltaic power plant may berepresented by other types of signals with appropriate changes to therest of the photovoltaic power plant 200. For example, a voltage signalmay be represented by a current signal, and vice versa. As anotherexample, voltages and currents in the photovoltaic power plant may berepresented in RMS (root mean square).

An unloading deadband function 302 may or may not be enabled dependingon the application. The unloading deadband function 302 allows the errorvoltage V_(ERR) to vary within a range without adjusting the controlsignal to the inverters 110. More specifically, the unloading deadbandfunction 302 allows the input to the compensator 303 to vary up or downwhen the grid voltage (i.e., the voltage at the POI 204) is withinbounds (typically ±1% of nominal), and keep the inverters 110 at asetting such that the inverters 110 export a unity power factor. Inother words, if the grid voltage is within ±1%, for example, the inputto the compensator 303 is actually just the grid voltage. This will alsocause the inverters 110 to go to unity power factor if the grid voltageis within the deadband limits.

In one embodiment, a compensator 303 generates a global inverterreactive power setpoint signal from the error voltage V_(ERR) using aproportional-integral (PI) control scheme. Other control schemes thatmay be used include proportional, integral, derivative,proportional-integral, integral-derivative, proportional-derivative, andproportional-integral-derivative. The PI compensator 303 may beabsolute, which means the inverter reactive power setpoint is biased upor down based on the error signal V_(ERR) and the proportional gain (Kp)and integral gain (Ki) of the PI compensator 303. The compensator 303may also be incremental. The compensator 303 may have integral windupprotection and saturation limits. The compensator 303 may be enabled ordisabled due to state machine logic that is triggered when griddisturbances occur.

A reactive power limit select function 304 is configured to reduce orincrease the inverter reactive power setpoint signal output of thecompensator 303 based on inputs from the inverter voltage limitersub-loop 330, power factor limiter sub-loop 331, and reactive powerlimiter sub-loop 332. The reactive power limit select function 304adjusts the inverter reactive power setpoint signal such that aninverter 110 does not generate an output that exceeds a voltage limit(V_(INV) limit), a power factor limit (PF limit) and a reactive powerlimit (Q limit).

The inverter voltage limiter sub-loop 330 is configured to limit theinverter reactive power setpoint based on the voltage output at theterminals of the inverters 110. In one embodiment, the terminal voltagesof all the inverters 110 in the photovoltaic power plant 200 areaveraged together to generate a single inverter terminal voltagerepresenting the terminal voltage of all the inverters 110. In anotherembodiment, the highest inverter terminal voltage among all theinverters 110 is used to represent the terminal voltage of all theinverters 110. The inverter terminal voltage (averaged, highest, orother representation of terminal voltages of the inverters 110) isfiltered by a voltage filter 341 and compared to an inverter referencevoltage limit V_(INV) limit by a summer 309. The output of the summer309 is input to the compensator 308, which provides its output to thereactive power limit select function 304. Unlike the power factor andreactive power sub-loops, the inverter voltage limiter sub-loop 330 getsits measurement directly from the inverters 110 (not from the meter205). The inverter voltage limiter sub-loop 330 has its own compensator308 because the voltage at the POI 204 is not necessarily the same asthe voltage at the inverter terminals due to impedance changes with realpower flowing through the plant AC collection system. The compensator308 may employ a PI or other control scheme.

The power factor limiter sub-loop 331 is configured to limit theinverter reactive power setpoint when the power factor measured by themeter 205 at the POI 204 is close to, at, or over a power factor limit(PF Limit) of the photovoltaic power plant 200. The power factor readingfrom the meter 205 is filtered by a power factor filter 342 and comparedto the power factor limit by a summer 313. The difference between thepower factor reading from the meter 205 and the power factor limit isinput to the compensator 310, which provides its output to the reactivepower limit select function 304. The power factor limiter sub-loop 331has its own compensator 310 because the power factor at the POI 204 isnot necessarily the same as the power factor at the inverter terminalsdue to impedance changes with real power flowing through the plant ACcollection system. The compensator 310 may employ a PI or other controlscheme.

The reactive power limiter sub-loop 332 is configured to limit theinverter reactive power setpoint when the reactive power measured by themeter 205 at the POI 204 is close to, at, or over a reactive power limit(Q Limit) of the photovoltaic power plant 200. The reactive powerreading from the meter 205 is filtered by a reactive power filter 343and compared to the reactive power limit by a summer 312. The differencebetween the reactive power reading from the meter 205 and the reactivepower limit is input to the compensator 311, which provides its outputto the reactive power limit select function 304. The reactive powersub-loop 332 has its own compensator 311 because reactive power factorat the POI 204 is not necessarily the same as at the reactive power atthe inverter terminals due to impedance changes with real power flowingthrough the plant AC collection system. The compensator 311 may employ aPI or other control scheme.

A reactive power rate of change limit function 305 limits the rate ofchange of the inverter reactive power setpoint signal. This protectsagainst rapid and drastic changes to the inverter reactive powersetpoint.

An inverter available function 306 periodically receives heartbeatsignals 340 to detect inverter outages. Only one heartbeat signal 340from one inverter 110 is shown in FIG. 3 for clarity of illustration. Inpractice, a separate heartbeat signal 340 may be received from eachinverter 110. For each available inverter 110, the inverter availablefunction 306 outputs a corresponding inverter reactive power setpointsignal to an individual reactive power rate of change limit function307.

The individual reactive power rate of change limit function 307 isapplied to each individual inverter reactive power setpoint signal thatis provided to a corresponding inverter 110. In one embodiment, aninverter reactive power setpoint signal (Inv Q SP) is provided to acorresponding inverter 110 by way of a ModbusTCP register. An inverterreactive power setpoint signal is read from the register and convertedto an analog voltage signal, which is then presented to a terminal ofthe inverter 110 at the inverter pad 220 where the inverter 110 islocated

The individual reactive power rate of change limit function 307 is alsoconfigured to ramp an inverter reactive power setpoint up or down inresponse to inverter outages. For example, if an inverter 110 is offline(e.g., missing a heartbeat), the function 307 may set the inverterreactive power setpoint for that inverter 110 to unity power factor orzero reactive power. When that inverter 110 goes back online, thefunction 307 may set the inverter reactive power setpoint for thatinverter 110 back to the global reactive power setpoint as dictated byAVR.

FIG. 4 shows a state diagram of the state machine 400 for automaticvoltage regulation of a photovoltaic power plant in accordance with anembodiment of the present invention. The plant controller 202 (see FIG.2) may perform the actions dictated by the state machine 400.

In the state machine 400, the states 423, 424, 425, 426, 427, and 429represent a condition where the AVR is on but voltage control is lostdue to the photovoltaic power plant 200 reaching a reactive power, powerfactor, voltage, or minimum real power generation limit. The states 470and 480 represent a condition where the reactive power setpoint of thephotovoltaic power plant 200 (i.e., reactive power setpoint for theentire power plant) and of the inverters 110 are manually set, there isno closed loop control, and AVR is turned off. The states 421, 422, and430 represent a condition where a momentary (in the case of states 421and 430) or persistent (in the case of the state 422) anomalouscondition is detected, and the last value of the global inverterreactive power setpoint is held.

The state machine 400 begins from an initial state 410, which is thedefault state when the state machine 400 is initialized. From theinitial state 410, the state machine 400 transitions to the transitionto run state 429. In the state 429, proportional-integral-derivative(PID), or other control schemes, of the photovoltaic power plant 200performs a scan cycle to initialize. In one embodiment, the PID controlscheme is employed in states 420, 423, 426, 427, and 425. Inputparameters are also checked in the state 429 to validate that AVR can beenabled.

From the state 429, the state machine 400 transitions to the AVR state420, where closed loop control is enabled and working correctly, and thegrid voltage is within a setpoint deadband. More particularly, in thestate 420, AVR is enabled and running in closed loop control. In thestate 420, the AVR control scheme controls the power plant and inverterreactive power setpoints to control the voltage at the POI 204. Asetpoint deadband may allow the grid voltage measured at the POI 204 tovary +/−1%, for example, before the inverters 110 are commanded toprovide reactive power. From the state 420, the state machine 400 maytransition to other states depending on conditions of the photovoltaicpower plant 200.

The state machine 400 transitions from the AVR state 420 to the manualinverter setpoint state 470 and to the manual plant setpoint state 480when AVR is turned off and not enabled. Instead, the inverters 110(state 470) and the photovoltaic plant 200 (state 480) are commanded toa specific reactive power setpoint, which may indicate a specificreactive power in kVAR. In one embodiment, bumpless transfer is usedwhen switching modes, i.e., transitioning from AVR control (state 420)to manual control (states 470 and 480) of reactive power setpoints. Toget back to AVR mode, the state machine 400 transitions from states 470and 480 to the transition to run state 429, and then to the AVR state420.

The state machine 400 transitions from the AVR state 420 to the failsafe state 421 upon occurrence of momentary loss of communications withthe POI meter, which in this example is the meter 205. In the state 421,the AVR routine holds the global inverter reactive power setpoint at itslast position (i.e., last value). The state machine 400 automaticallytransitions back to the state 420 to resume AVR when the communicationsproblem with the POI meter is resolved.

The state machine 400 transitions from the AVR state 420 to the failsafe state 430 upon occurrence of momentary erroneous POI meterreadings. For example, the transition from the state 420 to the state430 may occur when the meter 205 gives suspect real power, reactivepower, power factor, or current readings, such as when the meter 205gives readings that are out of allowable range, for a brief moment. Inthe state 430, the automatic voltage regulation routine holds theinverter reactive power setpoints at their last position. The statemachine 400 automatically transitions back to the state 420 to resumeAVR when the problem with the readings of the meter 205 clears.

The state machine 400 transitions from the AVR state 420 to the failsafe state 422 upon occurrence of persistent erroneous POI meter-relatedproblems, such as persistent erroneous POI meter readings or persistentPOI meter communications failure. For example, the transition from thestate 420 to the state 422 may occur when the meter 205 persistentlygives suspect real power, reactive power, power factor, or currentreadings, such as when the meter 205 gives readings that are out ofallowable range. As another example, the state machine 400 maytransition from the state 420 to the state 422 when communications withthe meter 205 cannot be established for more than a predetermined amountof time. Since the state 422 is for a condition where there is apersistent, as opposed to momentary or temporary, POI meter relatedproblem, the AVR routine is configured to ramp the global inverterreactive power setpoint to unity power factor. The state machine 400automatically transitions back to the state 420 to resume automaticvoltage regulation when the problem with the POI meter clears.

The state machine 400 transitions from the AVR state 420 to the AVRpower factor limit state 426 when the power factor of the photovoltaicpower plant 200 exceeds the maximum allowable power factor PFmax asmeasured by the POI meter, i.e., at the POI 204. The state 426 reducesthe power factor of the power plant 200 to a value that is lower thanthe maximum allowable power factor PFmax. The power factor of thephotovoltaic power plant 200 may be detected and processed by way of thepower factor limiter sub-loop 331 (see FIG. 3). In the state 426, theinverter reactive power setpoints may be held steady or modified tobring the plant power factor value to within allowable limits. The statemachine 400 automatically transitions back to the state 420 when thepower factor of the photovoltaic power plant 200 returns to allowablelimits.

The state machine 400 transitions from the AVR state 420 to the AVRreactive power limit state 423 when the reactive power exported by thephotovoltaic power plant 200 as measured by the POI meter exceeds themaximum allowable reactive power Qmax. The state 423 reduces thereactive power of the plant to a value that is lower than the maximumallowable reactive power Qmax. The reactive power exported by thephotovoltaic power plant 200 may be detected and processed by way of thereactive power limiter sub-loop 332 (see FIG. 3). In the state 423, theinverter reactive power setpoints may be held steady or modified tobring the plant reactive power value within allowable limits. The statemachine 400 automatically transitions back to the state 420 when thereactive power exported by the photovoltaic power plant 200 returns toallowable limits.

The state machine 400 transitions from the AVR state 420 to the AVRinverter voltage limit state 427 when the inverter terminal voltage(e.g., average of the voltages at the terminals of the inverters 110)reaches the maximum allowable terminal voltage. The inverter terminalvoltage may be detected and processed by way of the voltage limitersub-loop 330 (see FIG. 3). In the state 427, the inverter reactive powersetpoints may be held steady or modified to bring the inverter terminalvoltage to within allowable limits. The state machine 400 automaticallytransitions back to the state 420 when the inverter terminal voltagereturns to allowable limits.

The state machine 400 transitions from the AVR state 420 to the AVR realpower generated cutout state 424 when the real power generated by thephotovoltaic power plant 200 goes below a minimum real power generationlevel. The real power generated by the photovoltaic power plant 200 maybe read by the plant controller 202 from the POI meter (see meter 205,FIG. 2). In the state 424, the inverter reactive power setpoints areramped gradually to a unity setpoint. The state machine 400 remains inthe state 424 until real power generation levels rise above an enablinglevel that is higher than a minimum real power resume generation levelfor hysteresis.

The state machine 400 transitions from the AVR state 420 to the AVR rampup state 425 when the state machine 400 was previously in the state oflow real power generation state (i.e., state 424), the real powergenerated by the photovoltaic power plant 200 rises above the enablinglevel, and the inverter reactive power setpoints now ramp up to theappropriate values for AVR control. The state machine 400 automaticallytransitions from the state 425 to the state 420 when the real powergenerated by the photovoltaic plant 200 rises above the minimum realpower generation level.

FIGS. 5-7 show a flow diagram of a method of controlling operation of aphotovoltaic power plant in accordance with an embodiment of the presentinvention. The flow diagram of FIGS. 5-7 show details of the operationof the state machine 400 as performed by the plant controller 202.

Beginning with FIG. 5, the state machine 400 begins in the initial state410. When the state machine 400 receives a command to go to AVR mode(block 503), the state machine 400 transitions to the transition to runstate 429. The plant operator may also issue a command to turn on AVR.Prior to transitioning from the state 429 to the AVR state 420, a seriesof checks are performed to validate that AVR can be enabled.

In the transition to run state 429, the state machine 400 checks for acommand to run in manual inverter setpoint mode. If there is a commandto run in manual plant reactive power setpoint mode (block 504), thestate machine 400 transitions to the manual plant setpoint state 480,and stays in the state 480 until a command to go to automatic voltageregulation mode is received (block 506). Similarly, if there is acommand to run in manual inverter reactive power setpoint mode (block505), the state machine 400 transitions to the manual inverter setpointstate 470, and stays in the state 470 until a command to go to automaticvoltage regulation mode is received (block 507).

The communications with and readings of the POI meter are checked forproper operation (node “D” of FIG. 5 to node “D” of FIG. 6). The statemachine 400 may enter the fail safe state 422 when the POI meterreadings (e.g., real power, reactive power, power factor, and current)are persistently bad (block 551), such as when the readings are bad fora time greater than 5 seconds, or when communications with the POI meteris lost and cannot be reestablished within a predetermined period oftime. The state machine 400 enters the fail safe state 430 when the POImeter readings are momentarily bad (block 552), such as when thereadings are bad for a time less than 5 seconds. The state machine 400enters the fail safe state 421 when communications with the POI meterare momentarily lost (block 553). The state machine 400 returns to thebeginning of the transition to run state 429 when these POI meterrelated problems are cleared (blocks 554, 555, and 556; node “B” of FIG.6 to node “B” of FIG. 5).

The state machine 400 may include a setpoint deadband (block 701; node“E” of FIG. 6 to node “E” of FIG. 7). In one embodiment, the setpointdeadband allows the grid voltage measured at the POI 204 to vary +/−1%,for example, before the inverters 110 are commanded to provide reactivepower.

The real power generated by the photovoltaic power plant 200 is comparedagainst the minimum real power generation level (block 702). When thereal power generated by the photovoltaic power plant 200 reaches theminimum real power generation level, the state machine 400 enters theAVR real power generation cutout state 424 to ramp down the inverterreactive power setpoints to unity. When the real power generated by thephotovoltaic system 200 rises above an enabling level (block 706), thestate machine 400 enters the AVR ramp up state 425 to ramp up theinverter reactive power setpoints to appropriate values for AVR control(block 707). The state machine 400 then returns to the beginning of thetransition to run state 429 (node “A” of FIG. 7 to node “A” of FIG. 5).

The state machine 400 enters the AVR inverter voltage limit state 427when the average terminal voltage of the inverters 110 exceeds themaximum allowable terminal voltage (block 703). In the example of FIG.7, the average of the terminal voltages of the inverters 110 (Vinv_avg)is considered the inverter terminal voltage for comparison to themaximum allowable terminal voltage Vmax. The state machine 400 entersthe AVR reactive power limit state 423 when the reactive power exportedby the photovoltaic power plant 200 exceeds the maximum allowablereactive power Qmax (block 704). The state machine 400 enters the AVRpower factor limit state 426 when the power factor of the photovoltaicpower plant 200 exceeds the maximum allowable power factor PFmax (block705). The state machine 400 returns to the beginning of the transitionto run state 429 when these conditions are cleared (blocks 708, 709, and710; node “A” of FIG. 7 to node “A” of FIG. 5).

From the transition to run state 429 (see FIG. 5), the state machine 400transitions to the AVR state 420 when the above-described series ofchecks validate that automatic voltage regulation mode can be entered.From the automatic voltage regulation mode of state 420, the statemachine 400 may enter other states depending on subsequent condition ofthe photovoltaic power plant 200.

FIG. 8 shows a flow diagram of a method 800 of controlling operation ofa photovoltaic power plant in accordance with an embodiment of thepresent invention. In the example of FIG. 8, the method 800 comprisesoperations of the state machine 400 involving the states 424 and 425. Inthe method 800, the real power generated by the photovoltaic power plant200 is compared against a trigger point, which is the minimum real powergeneration level in the example of FIG. 8 (block 702; see also FIG. 7).When the real power generated by the photovoltaic power plant 200 isbelow the minimum real power generation level Pmin as measured by thePOI meter, the state machine 400 enters the AVR real power generatedcutout state 424 to ramp down the inverter reactive power setpoints tounity power factor (i.e., 1.0 PF or zero kVar) (block 424). When thereal power generated by the photovoltaic system 200 rises above theenabling level Presume (block 722), the state machine 400 enters the AVRramp up state 425 to ramp up the inverter reactive power setpoints toappropriate values for AVR control. In one embodiment, the ramp up ofinverter reactive power setpoints under state 425 is performed for atleast 8 minutes (block 723), for example, to reach steady state beforethe photovoltaic power plant 200 is placed in AVR mode by entering theAVR state 420. The ramp up period interval may be optimized forparticular photovoltaic power plants based on power plant generationsize, reactive power needs, or other power plant and regioncharacteristics.

The method 800 addresses a problem that concerns photovoltaic powerplants. AVR by itself may not be able to regain tight control of the POIvoltage when certain transient conditions occur. For example, in themorning when the inverters come online, the inverters may awake fromsleep mode because enough DC (direct current) voltage is present toexport power. However, when the inverters start gating and exportingpower under this condition, the inverters may sense there is not enoughpower to export and would consequently shut down. The wake up andshutdown cycle may repeat for one or more inverters several times, andmay perturb AVR. In the method 800, soft starting is performed by notentering AVR until the inverters can generate sufficient real power(Presume) as measured at the POI in the case of the method 800 (seeblocks 702 and 722) or at the inverter terminals. Once the invertersgenerate real power above the sufficient real power level, the inverterreactive power setpoints, which is a controlled variable of the AVR, maybe smoothly ramped up in linear fashion to their final values asdetermined by the AVR control scheme. This allows the photovoltaic powerplant to come online during the morning or from plant shutdowns in asmooth manner that is less likely to disturb the power grid or powerplant operation.

Similarly, when the photovoltaic power plant is about to shutdown in theevening or for a scheduled shutdown (e.g., curtailment command), AVR maybe soft stopped by ramping down in linear fashion the inverter reactivepower setpoints to zero reactive power (0 kVar) or unity power factorover a defined period interval, which in the example of FIG. 8 is 8minutes (see block 723). The period interval may be optimized forparticular photovoltaic power plants based on power plant generationsize, reactive power needs, or other power plant and regioncharacteristics. Appropriate deadband may be configured between the softstart and soft stop thresholds such that the photovoltaic power plantdoes not inadvertently shutdown or startup repeatedly due to, forexample, irradiance induced power fluctuations, invertershutdown/startups, breaker trips, and so on.

In the method 800, the inverter reactive power setpoints are ramped tosoft start or soft stop the AVR. In other embodiments, the power factorsetpoints are ramped up to their final destination to soft start the AVRor ramped down to unity power factor or zero reactive power (0 kVar) tosoft stop the AVR.

The above-described soft starting and soft stopping techniques areexplained in terms of AVR as the primary control loop. Other primarycontrol loops that may also benefit from the soft starting and softstopping techniques include power factor control and reactive powercontrol.

FIG. 9 shows a flow diagram of a method 900 of controlling operation ofa photovoltaic power plant in accordance with an embodiment of thepresent invention. In the example of FIG. 9, the method 900 comprisesoperations of the state machine 400 involving the state 423. In the flowdiagram of FIG. 9, the state machine 400 enters the AVR reactive powerlimit state 423 when the reactive power exported by the photovoltaicpower plant 200, e.g., as measured at the POI 204, exceeds the maximumallowable reactive power Qmax (block 704; see also FIG. 7). Otherwise,the state machine 400 enters the AVR sate 420 assuming all otherconditions for the AVR state 420 are satisfied. The PI compensator 311(“POI Q PI Compensator” in block 732) of the sub-loop 332 (see FIG. 3)receives both the reactive power limit (“IN: SP=Qlimit” in block 732)and the POI meter reactive power reading (“IN: PV=POI_Q”). The outputQ_Qlimit of the PI compensator 311 (“Out: Q_Qlimit=PID_out” in block732) is input to the reactive power limit select function 304, whichoutputs the inverter reactive power setpoint as the lower of thesetpoint Q_AVR set by the AVR and the output of Q_Qlimit of the PIcompensator 311 (block 733).

FIG. 10 shows a flow diagram of a method 950 of controlling operation ofa photovoltaic power plant in accordance with an embodiment of thepresent invention. In the example of FIG. 10, the method 950 comprisesoperations of the state machine 400 involving the state 426. In the flowdiagram of FIG. 10, the state machine 400 enters the AVR power factorlimit state 426 when the power factor of the photovoltaic power plant200 exceeds the maximum allowable power factor PFmax (block 705; seealso FIG. 7). Otherwise, the state machine 400 enters the AVR sate 420assuming all other conditions for the AVR state 420 are satisfied. ThePI compensator 310 (“POI PF PI Compensator” of block 742) of thesub-loop 331 (see FIG. 3) receives both the power factor limit (“IN:SP=PFlimit) and the POI meter power factor reading (“IN: PV=POI_PF”).The output Q_PFlimit of the PI compensator 311 (“Out: Q_PFlimit=PID_out”in block 742) is input to the reactive power limit select function 304,which outputs the inverter reactive power setpoint as the lower of thesetpoint Q_AVR set by the AVR and the output Q_PFlimit of the PIcompensator 311 (block 743).

Often, a photovoltaic power plant is not required to export or absorball the reactive power that the inverters are capable of exporting orabsorbing. Sometimes, a contractual agreement may indicate what reactivepower and/or power factor the photovoltaic power plant must be capableof exporting or absorbing. If the photovoltaic power plant allows thefull export/absorb of inverter reactive power, the inverters may curtailreal power output of the photovoltaic power plant due to the inverters'apparent power limit. The methods 900 and 950 address these problemsassociated with photovoltaic power plants by dynamically limiting thepower plant reactive power in the case of the method 900 and powerfactor in the case of the method 950. The limits are dynamic in thatthey utilize a process value measurement from the point ofinterconnection (or some other location) instead of using the capabilityof the inverters. The dynamic limits may account for changes in reactivepower exporting/absorbing that occurs within the photovoltaic generationfacility AC collection, transformers, filters, switching elements, etc.

For example, in the case of the method 900, if the reactive powergenerated by the power plant exceeds the maximum allowable reactivepower for the power plant, a lower reactive power limit is incurred onthe inverter reactive power setpoints. The lower reactive power limit onthe inverter reactive power setpoints brings the power plant reactivepower limit down to a desired limit. If the magnitude of the reactivepower generated by the power plant is below the maximum allowablereactive power for the power plant, the lower reactive power limitimposed in the state 423 on the inverter reactive power setpoints isremoved by going back to AVR mode in state 420. The lower reactive powerlimit imposed on the inverter reactive power setpoints in the state 423may also be raised slightly to stay in the state 423 when the magnitudeof the plant reactive power has a value that is very close to orapproximately the same as the lower reactive power limit in the state423 to allow the inverters to operate near the plant reactive powerlimit.

Similarly, in the case of the method 950, a limit is incurred on theinverter power factor setpoints when the plant power factor exceeds themaximum allowable power factor. The lower inverter power factor limit onthe inverter power factor setpoints brings the plant power factor downto a desired limit. If the magnitude of the plant power factor is belowthe maximum allowable plant power factor, the lower inverter powerfactor limit imposed on the plant power factor setpoint in the state 426is removed by going back to AVR mode in state 420. The lower plant powerfactor limit imposed on the inverter power factor setpoints in the state426 may also be raised slightly to stay in the state 426 when themagnitude of the plant power factor has a value that is very close to orapproximately the same as the lower plant power factor limit in thestate 426 to allow the inverters to operate near the plant power factorlimit.

The dynamic limits of the methods 900 and 950 may be imposed andcontrolled at the same time. Furthermore, while the methods 900 and 950are described in terms of AVR, the methods 900 and 950 may also beapplied to automatic power factor control, reactive power control,emergency VAR support control, and other control schemes.

FIG. 11 shows a flow diagram of a method of controlling operation of aphotovoltaic power plant in accordance with an embodiment of the presentinvention. The flow diagram of FIG. 11 shows further details of thestate machine 400 involving the state 427. In the flow diagram of FIG.11, each inverter 110 that is communicating properly with the plantcontroller 202 (block 751) is included in the group of inverters 110whose terminal voltages are averaged together to form an inverterterminal voltage signal (block 752). The state machine 400 enters theAVR inverter voltage limit state 427 when the average terminal voltageof the inverters 110 exceeds the maximum allowable terminal voltage Vmax(block 703; see also FIG. 7). Otherwise, the state machine 400 entersthe AVR sate 420 assuming all other conditions for the AVR state 420 aresatisfied. The PI compensator 308 (“POI Inv term V PI Compensator” ofblock 754) of the sub-loop 330 (see FIG. 3) receives both the inverterreference voltage limit (“IN: Vinv=Vinv_limit) and the average inverterterminal voltage (“IN: PV=Vinv_avg”). The output Q_Vlimit of the PIcompensator 308 (“Out: Q_Vlimit=PID_out” in block 754) is input to thereactive power limit select function 304, which output the inverterreactive power setpoint as the lower of the setpoint Q_AVR set by theAVR and the output Q_Vlimit of the PI compensator 308 (block 755).

An improved automatic voltage regulation technique for photovoltaicpower plants has been disclosed. While specific embodiments of thepresent invention have been provided, it is to be understood that theseembodiments are for illustration purposes and not limiting. Manyadditional embodiments will be apparent to persons of ordinary skill inthe art reading this disclosure.

1. A method of controlling operation of a photovoltaic power plant, themethod comprising: generating direct current using a plurality of solarcells; converting the direct current generated by the solar cells toalternating current using a photovoltaic inverter; coupling an output ofthe photovoltaic inverter to a power grid; detecting that thephotovoltaic power plant generates a photovoltaic power plant outputthat exceeds a maximum allowable output limit of the photovoltaic powerplant; and in response to detecting that the photovoltaic power plantgenerates a photovoltaic power plant output that exceeds the maximumallowable output limit of the photovoltaic power plant, lowering asetpoint variable limit of the photovoltaic inverter from a firstinverter setpoint limit to a second inverter setpoint limit.
 2. Themethod of claim 1 wherein the photovoltaic power plant output comprisesreactive power exported or absorbed by the photovoltaic inverter, themaximum allowable output limit of the photovoltaic power plant comprisesa maximum allowable reactive power exported by the power plant, and eachof the first and second inverter setpoint limits comprises a reactivepower setpoint limit of the inverter.
 3. The method of claim 1 whereinthe detecting that the photovoltaic power plant generates thephotovoltaic power plant output that exceeds the maximum allowableoutput limit of the photovoltaic power plant is performed in a firststate of photovoltaic power plant operation and the lowering thesetpoint variable limit of the photovoltaic inverter from the firstinverter setpoint limit to the second inverter setpoint limit isperformed in a second state of photovoltaic power plant operation. 4.The method of claim 3 further comprising, returning to the first stateof photovoltaic power plant operation in response to detecting that thephotovoltaic power plant output does not exceed the maximum allowableoutput limit of the photovoltaic power plant.
 5. The method of claim 4wherein the photovoltaic power plant is controlled by automatic voltageregulation (AVR) in the first state of photovoltaic power plantoperation.
 6. The method of claim 1 wherein the photovoltaic power plantoutput is detected at a point of interconnection (POI) to the powergrid.
 7. The method of claim 6 wherein the detecting that thephotovoltaic power plant generates the photovoltaic power plant outputthat exceeds the maximum allowable output limit of the photovoltaicpower plant is performed in a first state of photovoltaic power plantoperation, and a third state of operation is entered when communicationswith a meter detecting the photovoltaic power plant output at the POI islost.
 8. The method of claim 1 wherein the photovoltaic power plantoutput comprises the power factor of the photovoltaic power plant, themaximum allowable output limit of the power plant comprises a maximumallowable power factor of the photovoltaic power plant, and each of thefirst and second inverter setpoint limits comprises a power factorsetpoint limit of the inverter.
 9. A photovoltaic power plantcomprising: a plurality of solar cells; a photovoltaic inverterconfigured to convert direct current generated by the plurality of solarcells to alternating current; a meter detecting an output of thephotovoltaic inverter at a point of interconnection to a power grid; anda plant controller comprising a state machine, the plant controllerbeing configured to read the meter to detect the output of thephotovoltaic power plant at the point of interconnection in a firststate of operation and to dynamically adjust a setpoint variable limitof the photovoltaic inverter from a first inverter setpoint limit to asecond inverter setpoint limit when the output of the photovoltaic powerplant exceeds a maximum allowable output.
 10. The photovoltaic powerplant of claim 9 wherein the output of the photovoltaic power plantcomprises reactive power and the setpoint variable limit comprises aninverter reactive power setpoint.
 11. The photovoltaic power plant ofclaim 9 wherein the output of the photovoltaic power plant comprisespower factor and the setpoint variable limit comprises an inverter powerfactor setpoint.
 12. The photovoltaic power plant of claim 9 whereinfurther comprising a compensator configured to receive and processreadings from the meter.
 13. The photovoltaic power plant of claim 12wherein the compensator comprises a proportional-integral (PI)compensator.
 14. The photovoltaic power plant of claim 9 wherein thephotovoltaic power plant is controlled using automatic voltageregulation (AVR) in the first state of operation.
 15. (canceled) 16.(canceled)
 17. (canceled)
 18. A photovoltaic power plant comprising: aplurality of solar cells; a photovoltaic inverter configured to convertdirect current generated by the plurality of solar cells to alternatingcurrent; a meter configured to detect an output of the photovoltaicinverter at a point of interconnection to a power grid; and a plantcontroller configured to read the meter to detect a real power output ofthe photovoltaic power plant at the point of interconnection, to comparethe real power output of the photovoltaic power plant to a minimum realpower generation level, and to ramp down a reactive power setpoint ofthe photovoltaic inverter to unity power factor in response to detectingthe real power output of the photovoltaic power plant to be below theminimum real power generation level.
 19. The photovoltaic power plant ofclaim 18 wherein the plant controller is further configured to ramp upthe reactive power setpoint of the photovoltaic inverter in response todetecting that the real power output of the photovoltaic power plant isabove an enabling real power generation level.
 20. The photovoltaicpower plant of claim 19 further comprising a compensator configured toreceive and process readings from the meter.
 21. The photovoltaic powerplant of claim 20 wherein the compensator comprises aproportional-integral (PI) compensator.